Recent developments in the field of nonlinear optics are directed to prospective applications in optical information processing, telecommunications, and integrated optics.
Nonlinear optics deal with the interaction of light waves due to an electromagnetic field dependent susceptibility of an optically transparent substrate. Nonlinear optical effects are observed at light intensities which are sufficiently high that the electric field of the light waves is significant in comparison with the Coulomb electric field binding the electrons in the atoms and molecules of the light transmitting solid medium. Monochromatic light of the required intensity (e.g., 10.sup.7 V/cm) first became available with the discovery of the laser in 1960.
Laser frequency converters typically are based on inorganic crystals that respond nonlinearly to incident high-power optical radiation by changing the frequency of the radiation. Second harmonic generation (SHG) results when optical radiation passes through a transparent substrate having an electric susceptibility that is a nonlinear function of the intensity of the radiation. In principle, any optically transparent solid medium without inversion symmetry can produce second harmonic generation, providing that an electric field E of the electromagnetic radiation is sufficiently large.
Under certain conditions, the polarization of the electron distribution is proportional to the square of E, and a polarization wave is produced that is different from the input wave .omega.. The constant of proportionality is called the second order nonlinear susceptibility coefficient. Efficient second order generation depends not only upon this nonlinearity, but also upon matching the velocity of the fundamental wave .omega. and second harmonic 2.omega. of electromagnetic waves that propagate through the nonlinear medium.
Thus, a light transmitting solid medium must satisfy two structure requirements in order to achieve second harmonic generation and other second-order effects. First, the optical medium must not be symmetrical about a center point. The nonlinear second order susceptibility extinguishes in an optical medium that possesses a centrosymmetric structure. Second, for maximum second harmonic generation the optical medium must possess propagation directions whereby the optical medium birefringence cancels the natural dispersion, leading to a state of equal indicies of refraction at the fundamental and second harmonic frequencies. The phase matching can provide a high percentage of conversion of the incident light to the second harmonic wave.
In Laser Focus, vol. 18 (No. 2), February 1982, Garito et al describe optical nonlinearity in organic materials. Microscopic second order susceptibilities (.beta.) for aniline, nitrobenzene, p-nitrobenzene and 2-methyl-4-nitroaniline are evaluated. The advantages of organic polymers are discussed in comparison with other organic compounds employed in molecular crystals. Polymers exhibit high mechanical strength and chemical stability.
Attachment of an optically nonlinear molecular group to a polymer chain provides two desirable features, i.e., a large nonlinear optical susceptibility in a solid medium that is noncentrosymmetric. In addition, the highly anisotropic nature of polymer crystals tends to exhibit a natural birefringence, which facilitates phase matching in second harmonic generation.
U.S. Pat. No. 4,431,263 describes nonlinear optical materials based on polymerized diacetylenes. There is detailed elaboration of physical and theoretical principles which underlie nonlinear behavior in organic systems. Reference is made to Physical Review A, 20 (No. 3), 1179 (1979) by Garito et al, entitled "Origin of the Nonlinear Second-Order Optical Susceptibilities of Organic Systems".
Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington, D.C., 1983.
One aspect of nonlinear optical organic system research involves the investigation of external field induced dipolar alignment of molecules in a substrate such as a thin film. This has been demonstrated to occur for a guest dye in a polymer host when an electric field is applied to the organic mixture above the glass transition temperature of the polymer host. Noncentrosymmetric polar structures in organic crystals and molecular aggregates have been achieved in this manner. Other alignment methods are based on shear-induced uniaxial orientation of polymeric films and fibers.
Macromolecules, 15, 1385 (1982) by Meredith et al describes the preparation of films of a thermotropic liquid crystalline copolymer with mesogenic side chains doped (2% by weight) with the pleochroic dye 4-(dimethylamine)-4'-nitrostilbene (DANS). Orthoscopic examination of the films established that dc field induced alignment of the guest pleochroic dye molecules in the nematic polymeric host had been achieved, with about a 43.degree. alignment scatter around the alignment director. Second harmonic generation measurements were made to establish the value of second-order nonlinear susceptibility. The reduced dimensionality of the nematic environment enhanced the dc field induced statistical dipole alignment of the dopant molecule as predicted by the Ising model.
Polymer Communications, 24, 364(1983) by Shibaev et al describes the study of the electric field orientation of liquid crystalline polymers having mesogenic side groups. The molecular orientation induced in a heated liquid crystalline polymeric film by an electric field was retained by cooling the polymeric substrate below the glass transition temperature. The oriented state of the film was stable after the removal of the electric field. Transparent polyacrylate films with homeotropic orientation of mesogenic side groups were prepared in this manner.
U.S. Pat. No. 4,412,059 describes a polymeric material which is capable of forming high modulus, biaxially orientable structures of a cholesteric mesophase. Among the applications contemplated for the novel organic composition are polymeric sheets or films having controlled variations in crystallite orientation due to the application of an external magnetic or electric field to the cholesteric phase prior to solidification.
There is continuing research effort to develop new nonlinear optical organic systems for prospective novel phenomena and devices for laser modulation and deflection, information control in optical circuitry, light valves and optical switches. The potential utility of organic materials with large second-order nonlinearities for very high frequency application contrasts with the bandwidth limitations of conventional inorganic electrooptic materials.
Accordingly, it is an object of this invention to provide novel polymeric substrates having uniaxially oriented molecular structures with a high degree of main chain alignment.
It is another object of this invention to provide optically clear films of thermotropic liquid crystalline polymer which are adapted for second harmonic generation applications.
It is a further object of this invention to provide novel liquid crystalline polymeric films and coatings possessing a unique combination of mechanical properties which are adapted for high strength-low weight applications.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.